Laser diodes (hereinafter also referred to as diode lasers) generally have an active layer which is embedded between semiconductor layers, said semiconductor layers differing in their band-gaps, refractive indices and doping. The layers below and above the active layer differ in their conduction type (n or p). Apart from providing for the transport of electrons and holes to the active layer, where they recombine in an excited state and produce laser radiation, these layers support the vertical guiding of the laser light. The layers adjacent to the active layers are referred to as wave guiding layers, and the layers adjacent to said wave guiding layers are called cladding layers. The refractive index of the active layer is typically higher than the refractive index of the wave guiding layers, and the refractive index of the wave guiding layers is typically higher than the refractive index of the cladding layers. However, other configurations are possible as well (e.g. Vertical ARROW, Photonic Band Crystal).
Parameters such as power, efficiency, beam quality, narrow spectral linewidth and reliability are important criteria for using high-power diode lasers instead of solid-state lasers. Broad area single emitter lasers are diode lasers optimised for high optical output power. High output power is achieved by pumping large areas with apertures (stripe widths) larger than 10 μm, for example between 30 and 400 μm, and resonator lengths of up to 10 mm.
However, the use of broad area lasers with high output power instead of solid-state lasers is restricted by the lateral divergence of the emitted radiation, which increases with increasing output power. The focusability of the emitted radiation thus decreases, which is a disadvantage in many application situations, such as in materials processing. This effect is mainly due to the formation of a thermally induced waveguide which, with increased output power, leads to stable lasing of lateral modes of (ever) higher orders. Since the divergence in the far field increases with increasing order of the lateral modes, higher output power leads to an unwanted lateral widening of the beam, as FIG. 1 illustrates.
FIG. 1 shows the intensity distribution in the lateral far field of a conventional broad area laser with a stripe width of 90 μm for different levels of output power.
The laser is heated by the part of the current flowing into the laser and by the part of the emitted light which is absorbed again. The heating takes place in particular in the active region directly below the upper contact stripe through which current is flowing. This leads to a local increase in the refractive indices, since the refractive indices of the compound semiconductors used increase with increasing temperature at the laser wavelength. The positive difference of the refractive indices between the active region and the lateral passive region (which is not positioned directly below the contact stripe) leads to the formation of a lateral waveguide that can carry several lateral modes. The difference of the refractive indices continues to increase with increasing output power, so that lateral modes of ever higher orders are carried, said lateral modes eventually reaching the lasing threshold.
To reduce the lateral far-field divergence, U.S. Pat. No. 4,965,806 suggests to imprint the upper waveguide with a lateral thickness variation to compensate for the increased refractive index. However, this is a disadvantage because the compensation created through the thickness variation can only be implemented for a particular output power. The high cost of implementing a non-planar waveguide is a further disadvantage.
In another embodiment, U.S. Pat. No. 4,965,806 suggests an imprinted lateral temperature profile created by heating elements arranged laterally to the active region. The disadvantage of this apparatus is that additional electrical connections are required. In addition, this apparatus is not compatible with p-down mounting for better heat dissipation. Furthermore, the high cost of the additional heating elements is a disadvantage.
To suppress higher order lateral modes, U.S. Pat. No. 6,141,365 and WO 2004/027951 A1 suggest absorption layers arranged laterally to the active region, said absorption layers leading to losses of higher order lateral modes. Such absorption layers, however, can be used sensibly only for small stripe widths <10 μm, i.e. for power <2 W, since the higher order lateral modes of broad area lasers are located primarily in the active region, which means that absorption layers arranged laterally can not, or only to a negligible degree, influence said lateral modes. Furthermore, such absorption layers can only be used for modes with strongly different lateral profiles, because the fundamental mode would otherwise also be weakened too much, which would lead to a low efficiency of the device.
WO 2010/057955 A1 uses lattice structures to decrease the feedback of higher order modes, but the manufacturing of said lattice structures is complex and hence expensive.